1. Field of the Invention
The present invention relates generally to the field of four wheel drive power transmissions for vehicles, and more particularly to such transmissions which are capable of transmitting power to a slower set of wheels, while permitting free overrun of the faster traveling set of wheels.
2. Description of the Prior Art
There are in the prior art four wheel drive vehicles where all four wheels of the vehicle are in positive engagement with a power transmission. While such an arrangement is effective for a vehicle which is traveling over difficult terrain, it does have shortcomings. For example, when the vehicle is turning so that the front turning wheels are traveling in a wider radius than the rear wheels, since the rotational speed of all the wheels is the same by virtue of all the wheels being locked into the transmission, there is a scrubbing of the wheels. Further, it often happens that the several wheels of the vehicle have slightly different diameters due to greater wear of some of the tires. Thus, when the vehicle has left the rough terrain and remains in four wheel drive as it is cruising along a highway, there will be a scrubbing of the tire surfaces on the highway. This is due to the different linear rates of travel of the wheels, which results from the wheels all having the same rotational speed but slightly different diameters.
Thus, drive mechanisms have been devised which supply drive to only one set of wheels (usually the rear wheels), for normal cruise conditions, where the four wheels of the vehicle are traveling at nearly the same speed. However, in a situation where the driving set of wheels begin to overrun the non-driving set of wheels by some particular margin, the non-driving wheels become engaged to the power transmission to make up for the traction lost by the wheels that normally drive the vehicle. Sometimes this is done by providing the non-driving set of wheels with an overrunning clutch, and gearing the clutch to become engaged when a predetermined speed differential between the driving and non-driving wheels occurs. The disadvantage of this system is that unless the overrunning clutch is manually locked up, no four wheel drive will be obtained in reverse or under conditions of engine compression braking. Other systems of "Full Time" four wheel drive utilize a conventional differential to allow the front and rear drive shafts to deliver power while rotating at different speeds. These systems generally require a manual lock up device to prevent excessive wheel spin when encountering non-tractive surfaces.
Representative of prior art in the filed of this invention are the following patents: U.S. Pat. No. 2,835,143, Kelbel; U.S. Pat. No. 2,959,237, Hill; U.S. Pat. No. 3,123,169 Young et al; U.S. Pat. No. 3,295,625, Ordorica et al; and U.S. Pat. No. 3,481,436, Wilkowski.
Also, as possible prior art, the applicant herein is aware of possible prior attempts to take a more or less conventional positive drive overrunning differential and connect this to distribute power between the front and rear wheels of a vehicle. (Such positive drive differentials supply power to the slower rotating wheel, while permitting the faster rotating wheel to overrun.) However, to the best knowledge of the applicant herein, such an arrangement has not been developed commercially. Also, it entails difficulty in locating the rear propeller shaft so that it can be connected to a centrally located rear wheel differential.
Also known in the prior art are clutching mechanisms applied to vehicle wheel hubs, which utilize an inner cammed member, roller clutch elements, and an outer generally cylindrical member capable of overrunning the inner member. Typical of such devices are those shown in U.S. Pat. No. 2,684,140; U.S. Pat. No. 2,884,101; U.S. Pat. No. 3,055,471, and U.S. Pat. No. 3,351,364. Also known in the prior art are various positive drive overrunning differential drive mechanisms, generally used to transmit power to the rear driving wheels of a vehicle. In such devices, there is a single driving member (usually an outer member) having cammed surfaces, two cylindrical driven members (usually inner members), and two sets of roller clutches for the two driven members. The roller clutch mechanisms are so arranged that the faster traveling of the two driven members is allowed to freely overrun, while power is delivered from the driving member to the slower moving driven member. Typical of such devices are those shown in U.S. Pat. No. 2,865,228; U.S. Pat. No. 2,966,075; U.S. Pat. No. 3,124,972; U.S. Pat. No. 3,173,309; U.S. Pat. No. 3,283,611; U.S. Pat. No. 3,447,396; U.S. Pat. No. 3,581,597; and U.S. Pat. No. 3,700,082. It is noted that the aforementioned U.S. Pat. No. 2,865,228, Weismann, discloses two configurations, one of which has a single outer driven member which drives one or both of the cylindrical inwardly positioned members, and a second configuration (shown in FIG. 5 of that patent) where there is a single inner driving member adapted to drive one or both of two outer driven members. This latter configuration is arranged to drive a pair of adjacent double tires, and yet permit overrun of either of the tires, for example in the situation where the vehicle to which its tires are mounted is making a turn.